1. Field of the Invention
The present invention relates to an electric submersible pump for circulating cooling water in water-cooled engines. More specifically, the present invention relates to an electric submersible pump having a sleeve bearing that has a small coefficient of friction in water.
2. Description of Related Art
A conventional water pump for pumping cooling water in a closed cooling water circuit is driven by a crank shaft of an engine. The cooling water circuit includes a water jacket of the engine connected to a radiator of the engine. Such a conventional pump's rotation corresponds to the number of revolutions of the engine. The number of revolutions of such pump could not be controlled in a fine manner. Furthermore, when the engine stops, the pump stops immediately thereby causing troubles.
On the other hand, if a pump for use with water is driven by an electric motor, it is possible to arbitrarily control the number of revolutions and keep it running even when the engine is stopped. It is also possible to arbitrarily control the flow volume of cooling water passing through a radiator by electrically varying the degree of opening of a thermostatically controlled valve. Such a cooling control device for an engine has been disclosed in Japanese Patent Application Laid-Open No. 5-231148.
The conventional electrically motorized pump for use with water has a structure in which the impeller side and the rotor side of a pump are sealed to prevent water from flowing through. An O-ring made of rubber is placed between the impeller side and the rotor side or a sealing material is allowed to be in close contact with a rotary shaft. When the rotor is used at high revolutions for a long period of time, the O-ring deteriorates causing a loss in energy, also the sealing material, which is in close contact with the shaft causes a loss in energy.
Ceramics such as silicon nitride and alumina, and super-engineering plastics such as PPS that are conventionally used as the materials for underwater sleeve bearings possess a combination of mechanical properties, chemical properties and physical properties that are required for bearings for electric submersible pumps. However, there is room for improvement in terms of friction characteristics, productivity, and cost.
It is an object of the present invention to provide a submersible sleeve bearing demonstrating excellent corrosion resistance and friction characteristics when used in liquids without sealing, for example, in cooling water of a water-cooled engine, which is a water-ethylene glycol mixture.
It is yet another object of the present invention to provide an electric submersible pump which requires no sealing between the impeller side and rotor side of the pump, allows water to flow freely therethrough, makes it possible to reduce energy consumption, and effectively carries out the circulation of cooling water in water-cooled engines.
It is still another object of the present invention to provide an electric submersible pump using a sleeve bearing that is fabricated by employing a material demonstrating remarkable sliding properties in water, this material also having a small energy loss, excellent corrosion resistance, and low friction.
The above objects are achieved by improving upon materials described in an article (Kinou Zairyou (Functional Materials), May 1997 issue, Vol. 17, No. 5, pp 24 to 28) by one of the inventors of the present case, Mr. Kazuo Hokkirigawa, that discloses a porous carbon material made by using rice bran.
The above-mentioned reference describes an RB ceramic (referred to hereinbelow as RBC) and a manufacturing method thereof. The RBC is a carbon-based material obtained by mixing defatted bran obtained from rice bran with a thermosetting resin, blending the mixture, press molding, drying the molding, and firing the dry molding in an inactive gas atmosphere. Any thermosetting resin may be used in this process. Typical examples of such resins include phenolic resins, diarylphthalate resins, unsaturated polyester resins, epoxy resins, polyamide resins, and triazine resins. The phenolic resins are preferred. The mixing ratio of the defatted bran and the thermosetting resin in the mixture is 50–90:50–10, preferably 75:25. The firing temperature is 700–1000° C. Usually the firing is conducted in a rotary kiln, and the firing time is from about 40 min to 120 min.
The carbonized rice bran ceramic (referred to hereinbelow as CRBC) is a carbon-based material obtained by modification of the RB ceramic. The carbonized rice bran ceramic is obtained by mixing defatted bran obtained from rice bran and a thermosetting resin, primary firing the mixture in an inactive gas at a temperature of 700–1000° C. grinding to a size of not more than about 100 mesh to obtain a carbonized powder, mixing the carbonized powder with a thermosetting resin, press molding under a pressure of 20–30 MPa, and heat treating the molding again at a temperature of 500–1100° C. in an inactive atmosphere.
RBC and CRBC have the following excellent characteristics:
    High hardness.    Oval shape even in the form of particles.    Very small expansion coefficient.    Porous structure.    Electric conductivity.    Low density and lightweight.    Very small friction coefficient.    Excellent wear resistance.    Small environmental impact because rice bran is used as a source material, leading to conservation of natural resources.